Mold-Mould | Commonly Confused Words

Mold

/məʊld/

A hollow form or pattern used to create shapes; it can also refer to a growth of fungi often found on old or damp materials.

/məʊld/

A hollow container used to give shape to something poured into it or a layer of fungus that grows on food or other surfaces.

Mold and mould mean the same thing, so differentiating between them primarily concerns spelling.
Mold is the preferred spelling in American English.
Mould is the preferred spelling in British English.
Both terms will be understood internationally, but using the American or British version indicates regional consistency.
If you're writing for a specific audience, choose the spelling that corresponds with their regional spelling norms.